1. Field of the Invention
The present invention relates to an electrolytically deposited hard chromium coating having a network of cracks extending through the entire thickness of the coating and to a method for manufacturing it. More particularly, the invention relates to a coating having solid particles deposited within the cracks and a method of manufacturing thereof.
2. Description of the Background
Electrolytically deposited hard chromium coatings evidence substantial hardness which results in high wear resistance, very smooth surfaces which result in a low coefficient of friction, and low adherence as well as good resistance to chemically aggressive, corrosive, erosive and oxidative stresses, even at higher temperatures.
Thus, objects like the bearing faces of machine parts from, e.g., piston rings or cylinders in internal-combustion engines, are coated with hard chromium layers in order to attain special wear resistance. Pressing tools and pressing molds for the manufacture of molded plastic articles are also routinely coated with chromium layers to attain special smoothness and wear resistance. Further, fittings primarily in chemical plants are also especially protected by chromium coatings to attain a longer service life.
During the electrolytic deposition of chromium, relatively high tensile stresses develop in the chromium coatings. Once a certain thickness of the layer is reached, the expansion capability of the weakly elastic chromium results in the formation of microcracks which, when observed in an etched surface micrograph, appear in the form of a spider web-like network of fine lines and cracks. Such microcrack networks serve as oil grooves or oil cups, and facilitate, in oil lubricated wear protection coatings, the wetting of the chromium coatings with oil and thus the formation of an oil film required for lubrication. For this purpose, and in order to enlarge the network of cracks in the chromium, a porous chromium coating is formed by selecting suitable chrome plating parameters during or after the chrome-plating process, periodic current reversals, thermal post-treatments or etching processes. On the other hand, in order to attain good corrosion protection chromium coatings should be as free of cracks as possible. For this purpose, a chrome plating process is usually selected in which the chromium coatings have at most a network of microcracks.
Moreover, it is known that hard chromium coatings have little elasticity and are brittle. As a consequence of this, and primarily under intermittent stresses and greater shocks, breaks may occur in the coatings which may cause chipping.
It is known to embed finely dispersed solids particles in a matrix metal wherein the metal deposition is effected from an electrolytic bath which has solids particles finely dispersed therein. This improves the physical characteristics of electrolytically deposited metal coatings. For example, nickel dispersion coatings with embedded hard substance particles, primarily of silicon carbide, exhibit improved wear resistance. These nickel dispersion layers with embedded solid lubricant particles evidence a lower coefficient of friction. While the production of electrolytically deposited nickel dispersion layers is relatively problem free, chromium dispersion layers cannot easily be produced. Consequently, electrolytically deposited chromium dispersion layers having improved characteristics are not being used in practice. In this case, dispersed solid particles are prevented from being deposited in the chromium coating by a more extensive development of hydrogen around the chromium atoms during the electroplating process. Therefore, hard chromium coatings containing finely dispersed solid particles must be manufactured by relatively complicated processes, such as plasma spraying.
Thus, there still a need for a hard chromium coating which has improved physical and technological characteristics while lacking the above-listed drawbacks. At the same time, there also still is a need for an electroplating process of broad application with which such coatings can be produced in a simple and economical manner.